We are engaged in a phylogenetic survey of invertebrate muscle to determine the distribution of paramyosin, using sodium-dodecyl-sulfate-polyacrylamide gel electrophoresis of homogenates of glycerinated contractile tissues. The amount of paramyosin present in muscle, relative to myosin heavy chains, is determined by quantitative SDS gel electrophoresis. Similarities among different paramyosins are studied by gel electrophoresis and immunological techniques such as immunodiffusion. Paramyosin is localized within cells at the light and electron microscopic level at different in situ muscle lengths, and also will be localized on isolated thick filaments.